Multiple band pass filtering for pyrometry in laser based annealing systems

ABSTRACT

A thermal processing system includes a source of laser radiation emitting at a laser wavelength, beam projection optics disposed between the reflective surface and a substrate support capable of holding a substrate to be processed, a pyrometer responsive to a pyrometer wavelength, and a wavelength responsive optical element having a first optical path for light in a first wavelength range including the laser wavelength, the first optical path being between the source of laser radiation and the beam projection optics, and a second optical path for light in a second wavelength range including the pyrometer wavelength, the second optical path being between the beam projection optics and the pyrometer. The system can further include a pyrometer wavelength blocking filter between the source of laser radiation and the wavelength responsive optical element.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.60/627,527, filed Nov. 12, 2004.

This application contains subject matter related to U.S. applicationSer. No. 11/185,454 filed Jul. 20, 2005entitled RAPID DETECTION OFIMMINENT FAILURE IN LASER THERMAL PROCESSING OF A SUBSRATE by BruceAdams, et al.; U.S. application Ser. No. 11/185,649 filed Jul. 20, 2005entitled SINGLE AXIS LIGHT PIPE FOR HOMOGENIZING SLOW AXIS OFILLUMINATION SYSTEMS BASED ON LASER DIODES by Bruce Adams, et al.; U.S.patent application Ser. No. 11/185,651 filed Jul. 20, 2005 entitledTHERMAL FLUX LASER ANNEALING FOR ION IMPLANTATION OF SEMICONDUCTOR P-NJUNCTIONS by Bruce Adams, et al.; and U.S. application Ser. No.11/198,660 filed Aug. 5, 2005 entitled AUTOFOCUS FOR HIGH POWER LASERDIODE BASED ANNEALING SYSTEM by Dean Jennings, et al., all of whichapplications are assigned to the present assignee.

FIELD OF THE INVENTION

The invention relates generally to thermal processing of semiconductorsubstrates. In particular, the invention relates to laser thermalprocessing of semiconductor substrates.

BACKGROUND OF THE INVENTION

Thermal processing is required in the fabrication of silicon and othersemiconductor integrated circuits formed in silicon wafers or othersubstrates such as glass panels for displays. The required temperaturesmay range from relatively low temperatures of less than 250° C. togreater than 1000°, 1200°, or even 1400° C. and may be used for avariety of processes such as dopant implant annealing, crystallization,oxidation, nitridation, silicidation, and chemical vapor deposition aswell as others.

For the very shallow circuit features required for advanced integratedcircuits, it is greatly desired to reduce the total thermal budget inachieving the required thermal processing. The thermal budget may beconsidered as the total time at high temperatures necessary to achievethe desired processing temperature. The time that the wafer needs tostay at the highest temperature can be very short.

Rapid thermal processing (RTP) uses radiant lamps which can be veryquickly turned on and off to heat only the wafer and not the rest of thechamber. Pulsed laser annealing using very short (about 20 ns) laserpulses is effective at heating only the surface layer and not theunderlying wafer, thus allowing very short ramp up and ramp down rates.

A more recently developed approach in various forms, sometimes calledthermal flux laser annealing or dynamic surface annealing (DSA), isdescribed by Jennings et al. in PCT/2003/00196966 based upon U.S. patentapplication Ser. No. 10/325,497, filed Dec. 18, 2002 and incorporatedherein by reference in its entirety. Markle describes a different formin U.S. Pat. No. 6,531,681 and Talwar yet a further version in U.S. Pat.No. 6,747,245.

The Jennings and Markle versions use CW diode lasers to produce veryintense beams of light that strikes the wafer as a thin long line ofradiation. The line is then scanned over the surface of the wafer in adirection perpendicular to the long dimension of the line beam.

SUMMARY OF THE INVENTION

A thermal processing system includes a source of laser radiationemitting at a laser wavelength, beam projection optics disposed betweenthe reflective surface and a substrate support capable of holding asubstrate to be processed, a pyrometer responsive to a pyrometerwavelength, and a wavelength responsive optical element having a firstoptical path for light in a first wavelength range including the laserwavelength, the first optical path being between the source of laserradiation and the beam projection optics, and a second optical path forlight in a second wavelength range including the pyrometer wavelength,the second optical path being between the beam projection optics and thepyrometer. The system can further include a pyrometer wavelengthblocking filter between the source of laser radiation and the wavelengthresponsive optical element. Preferably, the pyrometer includes aphotodetector and a laser wavelength blocking filter. In a preferredembodiment, the source of laser radiation includes an array of laseremitters and the pyrometer wavelength blocking filter comprises areflective surface angled relative to the array whereby to reflect lightof the pyrometer wavelength to zones between adjacent ones of theemitters. In a preferred embodiment, the beam projection optics projectsa line beam of radiation of the laser wavelength onto a substrate planeover the substrate support, and the system further includes a line beamscanning apparatus having a fast axis transverse to the line beam.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an orthographic representation of a thermal flux laserannealing apparatus employed in the present invention.

FIGS. 2 and 3 are orthographic views from different perspectives ofoptical components of the apparatus of FIG. 1.

FIG. 4 is an end plan view of a portion of a semiconductor laser arrayin the apparatus of FIG. 1.

FIG. 5 is an orthographic view of a homogenizing light pipe for theapparatus of FIG. 1.

FIG. 6 is a schematic diagram of a system including the features ofFIGS. 2-4 in accordance with a preferred embodiment.

FIGS. 7, 8 and 9 are elevational sectional views of differentembodiments of a pyrometer wavelength blocking optical filter of theinvention.

DETAILED DESCRIPTION OF THE INVENTION

One embodiment of the apparatus described in the above-referencedapplication by Jennings et al. is illustrated in the schematicorthographic representation of FIG. 1. A gantry structure 10 fortwo-dimensional scanning includes a pair of fixed parallel rails 12, 14.Two parallel gantry beams 16, 18 are fixed together a set distance apartand supported on the fixed rails 12, 14 and are controlled by anunillustrated motor and drive mechanism to slide on rollers or ballbearings together along the fixed rails 12, 14. A beam source 20 isslidably supported on the gantry beams 16, 18, and may be suspendedbelow the beams 16, 18 which are controlled by unillustrated motors anddrive mechanisms to slide along them. A silicon wafer 22 or othersubstrate is stationarily supported below the gantry structure 10. Thebeam source 20 includes a laser light source and optics to produce adownwardly directed fan-shaped beam 24 that strikes the wafer 22 as aline beam 26 extending generally parallel to the fixed rails 12, 14, inwhat is conveniently called the slow direction. Although not illustratedhere, the gantry structure further includes a Z-axis stage for movingthe laser light source and optics in a direction generally parallel tothe fan-shaped beam 24 to thereby controllably vary the distance betweenthe beam source 20 and the wafer 22 and thus control the focusing of theline beam 26 on the wafer 22. Exemplary dimensions of the line beam 26include a length of 1 cm and a width of 66 microns with an exemplarypower density of 220 kW/cm². Alternatively, the beam source andassociated optics may be stationary while the wafer is supported on astage which scans it in two dimensions.

In typical operation, the gantry beams 16, 18 are set at a particularposition along the fixed rails 12, 14 and the beam source 20 is moved ata uniform speed along the gantry beams 16, 18 to scan the line beam 26perpendicularly to its long dimension in a direction conveniently calledthe fast direction. The line beam 26 is thereby scanned from one side ofthe wafer 22 to the other to irradiate a 1 cm swath of the wafer 22. Theline beam 26 is narrow enough and the scanning speed in the fastdirection fast enough that a particular area of the wafer is onlymomentarily exposed to the optical radiation of the line beam 26 but theintensity at the peak of the line beam is enough to heat the surfaceregion to very high temperatures. However, the deeper portions of thewafer 22 are not significantly heated and further act as a heat sink toquickly cool the surface region. Once the fast scan has been completed,the gantry beams 16, 18 are moved along the fixed rails 12, 14 to a newposition such that the line beam 26 is moved along its long dimensionextending along the slow axis. The fast scanning is then performed toirradiate a neighboring swath of the wafer 22. The alternating fast andslow scanning are repeated, perhaps in a serpentine path of the beamsource 20, until the entire wafer 22 has been thermally processed.

The optics beam source 20 includes an array of lasers. An example isorthographically illustrated in FIGS. 2 and 3, in which laser radiationat about 810 nm is produced in an optical system 30 from two laser barstacks 32, one of which is illustrated in end plan view in FIG. 4. Eachlaser bar stack 32 includes 14 parallel bars 34, generally correspondingto a vertical p-n junction in a GaAs semiconductor structure, extendinglaterally about 1 cm and separated by about 0.9 mm. Typically, watercooling layers are disposed between the bars 34. In each bar 34 areformed 49 emitters 36, each constituting a separate GaAs laser emittingrespective beams having different divergence angles in orthogonaldirections. The illustrated bars 34 are positioned with their longdimension extending over multiple emitters 36 and aligned along the slowaxis and their short dimension corresponding to the less than 1-micronp-n depletion layer aligned along the fast axis. The small source sizealong the fast axis allows effective collimation along the fast axis.The divergence angle is large along the fast axis and relatively smallalong the slow axis.

Returning to FIGS. 2 and 3 two arrays of cylindrical lenslets 40 arepositioned along the laser bars 34 to collimate the laser light in anarrow beam along the fast axis. They may be bonded with adhesive on thelaser stacks 32 and aligned with the bars 34 to extend over the emittingareas 36.

The optics beam source 20 can further include conventional opticalelements. Such conventional optical elements can include an interleaverand a polarization multiplexer, although the selection by the skilledworker of such elements is not limited to such an example. In theexample of FIGS. 2 and 3, the two sets of beams from the two bar stacks32 are input to an interleaver 42, which has a multiple beam splittertype of structure and having specified coatings on two internal diagonalfaces, e.g., reflective parallel bands, to selectively reflect andtransmit light. Such interleavers are commercially available fromResearch Electro Optics (REO). In the interleaver 42, patterned metallicreflector bands are formed in angled surfaces for each set of beams fromthe two bar stacks 32 such that beams from bars 34 on one side of thestack 32 are alternatively reflected or transmitted and therebyinterleaved with beams from bars 34 on the other side of the stack 32which undergo corresponding selective transmission/reflection, therebyfilling in the otherwise spaced radiation profile from the separatedemitters 36.

A first set of interleaved beams is passed through a quarter-wave plate48 to rotate its polarization relative to that of the second set ofinterleaved beams. Both sets of interleaved beams are input to apolarization multiplexer (PMUX) 52 having a structure of a doublepolarization beam splitter. Such a PMUX is commercially available fromResearch Electro Optics. First and second diagonal interface layers 54,56 cause the two sets of interleaved beams to be reflected along acommon axis from their front faces. The first interface 54 is typicallyimplemented as a dielectric interference filter designed as a hardreflector (HR) while the second interface 56 is implemented as adielectric interference filter designed as a polarization beam splitter(PBS) at the laser wavelength. As a result, the first set of interleavedbeams reflected from the first interface layer 54 strikes the back ofthe second interface layer 56. Because of the polarization rotationintroduced by the quarter-wave plate 48, the first set of interleavedbeams passes through the second interface layer 56. The intensity of asource beam 58 output by the PMUX 52 is doubled from that of the eitherof the two sets of interleaved beams.

Although shown separated in the drawings, the interleaver 42, thequarter-wave plate 48, and the PMUX 52 and its interfaces 54, 56, aswell as additional filters that may be attached to input and outputfaces are typically joined together by a plastic encapsulant, such as aUV curable epoxy, to provide a rigid optical system. An importantinterface is the plastic bonding of the lenslets 40 to the laser stacks32, on which they must be aligned to the bars 34. The source beam 58 ispassed through a set of cylindrical lenses 62, 64, 66 to focus thesource beam 58 along the slow axis.

A one-dimensional light pipe 70 homogenizes the source beam along theslow axis. The source beam, focused by the cylindrical lenses 62, 64,66, enters the light pipe 70 with a finite convergence angle along theslow axis but substantially collimated along the fast axis. The lightpipe 70, more clearly illustrated in the orthographic view of FIG. 5,acts as a beam homogenizer to reduce the beam structure along the slowaxis introduced by the multiple emitters 36 in the bar stack 32 spacedapart on the slow axis. The light pipe 70 may be implemented as arectangular slab 72 of optical glass having a sufficiently high index ofrefraction to produce total internal reflection. It has a shortdimension along the slow axis and a longer dimension along the fastaxis. The slab 72 extends a substantial distance along an axis 74 of thesource beam 58 converging along the slow axis on an input face 76. Thesource beam 58 is internally reflected several times from the top andbottom surfaces of the slab 72, thereby removing much of the texturingalong the slow axis and homogenizing the beam along the slow axis whenit exits on an output face 78. The source beam 58, however, is alreadywell collimated along the fast axis (by the cylindrical lenslets 40) andthe slab 72 is wide enough that the source beam 58 is not internallyreflected on the side surfaces of the slab 72 but maintains itscollimation along the fast axis. The light pipe 70 may be tapered alongits axial direction to control the entrance and exit apertures and beamconvergence and divergence. The one-dimensional light pipe canalternatively be implemented as two parallel reflective surfacescorresponding generally to the upper and lower faces of the slab 72 withthe source beam passing between them.

The source beam output by the light pipe 70 is generally uniform. Asfurther illustrated in the schematic view of FIG. 6, further anamorphiclens set or optics 80, 82 expands the output beam in the slow axis andincludes a generally spherical lens to project the desired line beam 26on the wafer 22. The anamorphic optics 80, 82 shape the source beam intwo dimensions to produce a narrow line beam of limited length. In thedirection of the fast axis, the output optics have an infinite conjugatefor the source at the output of the light pipe (although systems may bedesigned with a finite source conjugate) and a finite conjugate at theimage plane of the wafer 22 while, in the direction of the slow axis,the output optics has a finite conjugate at the source at the output ofthe light pipe 70 and a finite conjugate at the image plane. Further, inthe direction of the slow axis, the nonuniform radiation from themultiple laser diodes of the laser bars is homogenized by the light pipe70. The ability of the light pipe 70 to homogenize strongly depends onthe number of times the light is reflected traversing the light pipe 70.This number is determined by the length of the light pipe 70, thedirection of the taper if any, the size of the entrance and exitapertures as well as the launch angle into the light pipe 70. Furtheranamorphic optics focus the source beam into the line beam of desireddimensions on the surface of the wafer 22.

A recurring problem with RTP and other types of radiant thermalprocessing is monitoring the temperature of the wafer being so thermallyprocessed. Temperature measurement is desired because the amount oflight coupled into the wafer strongly depends upon the surface structurealready formed in the wafer. Furthermore, light source conditions mayvary somewhat. Wide-angle pyrometers are generally used with RTP tomonitor large portions of the wafer. Such pyrometers are generallyinappropriate for focused laser beams irradiating only a small area ofthe wafer at any time, leaving the bulk of the wafer near ambienttemperature.

One aspect of the invention uses the same optics used to focus the lasersource light on the wafer to direct thermal radiation emitted from theneighborhood of the line beam 26 on the wafer 22 in the reversedirection to a pyrometer 60, schematically illustrated in FIG. 6,including an optical detector 61, such as a photodiode, and an opticalfilter 63 blocking the wavelength, e.g., 810 nm, of the laser radiation.The pyrometer filter 63 preferably is a narrow passband filter centeredat a region of the Plankian blackbody radiation curve which is quicklychanging at the temperatures of interest. A preferred pyrometer passbandis centered at 1550 nm (in which case the detector 61 may be an InGaAsphotodiode). As one of many alternatives, the pyrometer passband may becentered at 950 nm (in which case the detector 61 may be a Siphotodiode). The passband may extend over a few tens of nm for theshorter wavelength and perhaps 100 nm at the longer wavelength. Theoptics are generally reciprocal and thus in the reverse direction detectonly a small area of the wafer 22 on or very near to the line beam 26and optically expands that image to an area generally having a size ofthe emission face of the bar stacks. Although with small laser-beamannealing, there is significant variation of surface layer temperatureseven in the immediate neighborhood of the line beam 26, the nature ofthe Plankian blackbody spectrum causes the hottest areas to dominate thethermally emitted radiation. Typically narrow band interference filters63 are used having a passband near 1550 nm or 950 nm which block thelaser wavelength at 810 nm by several orders of magnitude andsubstantially block the laser spontaneous emission away from thepyrometer wavelength. The two PMUX interfaces 54, 56 are designed topass the pyrometer wavelength irrespective of its polarization. Such afunction can be implemented with interference mirrors tuned to the 810nm source radiation and not the pyrometer wavelength. The moreconventional interfaces 54, 56 are designed such that the firstinterface 54 is a hard-reflector (HR) interference filter but the secondinterface 56 is a polarization beam splitter (PBS) tuned to 810 nm orother laser wavelength. For the invention, the interference filter atthe PBS interface 56 is detuned to the extent that it passes asubstantial portion (e.g. 72%) of a first polarization at the pyrometerwavelength while reflecting the first polarization of the laser light(or at least most of it). Likewise, the interference filter in the HRinterface 54 is redesigned such that it passes a large fraction (e.g.,90%) of the first polarization at the pyrometer wavelength whilereflecting most of the other polarization of both wavelengths. As aresult of the filtering of both the PMUX interfaces 54, 56 and thenarrow band pyrometer filter 63, the photodetector 61 receives only thenarrow-band optical signal in a portion of the thermal (blackbodyradiation) spectrum. Thus, an optical filter or path that blocks oradmits a particular wavelength in the invention is sufficient eventhough it does not completely block or completely admit all of the lightat that wavelength.

The output of the photodetector 61 is supplied to a source controller65, which converts the detected photocurrent to a wafer temperature andcompares it to a desired temperature and thereby adjusts the powersupplied to the laser bars 32 to increase or decrease their opticaloutput in the direction of the desired wafer temperature.

A difficulty with this approach is that the GaAs or other semiconductorlasers have a fairly wide spectrum of low-level spontaneous emissionthat typically overlaps the pyrometer wavelength. As a result of thespontaneous emission, which the pyrometer filter 63 does not block atthe pyrometer wavelength, the photodetector 61 would detect both: (a)the wafer blackbody radiation at the pyrometer wavelength and (b) theportion of the laser source spontaneous emission at the pyrometerwavelength, in the absence of additional filtering.

The pyrometer performance can be greatly improved by filtering out thelaser source spontaneous radiation at the pyrometer wavelength with anotch filter 67 placed between the bar stacks 32 and the interleaver 42or a notch filter 68 placed between the interleaver 42 and the PMUX 52.The notch filter 67 or 68 blocks the source radiation at the pyrometerwavelength, specifically whatever wavelengths are passed by thepyrometer filter 63, e.g. 1550 nm or 950 nm, and pass at least the laserradiation at 810 nm. The ratio of the transmission coefficient of thelaser wavelength to that of pyrometer wavelength should be severalorders of magnitude. A minimum requirement of the filters 67, 68 is thatthey block wavelengths longer than the laser wavelength, e.g., 810 nm,although radiation at shorter wavelengths does not inherently degradethe pyrometer. The notch filters 67, 68 may be easily implemented asinterference filters coated on either the interleaver 42 or the PMUX 52,although they may be implemented as stand alone filters.

An advantageous form of the first notch filter 67, illustrated in crosssection in FIG. 7, includes a housing 79 in which each laser stack 32 isplaced and the lenslets 40 are bonded with adhesive to the laser stacksin over the emitting areas 36. The lenslets 40 are small cylindricallenses extending along the laser bars 34 extending perpendicularly tothe illustration to produce generally collimated beams. The top of thehousing 79 is sealed with a flat window or window slab 83 of Infrasil(optical quartz) on which an interference filter 84, corresponding topyrometer filter 67, is formed to selectively reflect the pyrometerwavelength and pass the laser wavelength. Anti-reflective coatings forthe laser radiation may be formed on either or both sides of the window83. The sealed structure has the advantage that if the lenslets 40delaminate from the laser stack 32 and the misalignment causes thermalrunaway at the laser source, the organic contamination is restricted tothe inside of the housing and does not affect the downstream optics.

However, the reflected radiation produces problems at the laser bars dueto both thermal heating and mode hopping. To solve this problem, wall86, 88 of the housing have unequal height so that the window 83 andattached filter 84 are tilted at about 1° or other appropriate anglerelative to a plane perpendicular to the incident beams 81. As a result,the reflected radiation at the pyrometer wavelength is reflected at anoblique angle in reflected beams 90 that strike the laser stacks 32 inzones between the emitting areas, that is, between the stacked bars 34.The radiation is thereby not coupled into the laser structure. A furtheradvantage of the tilted window 83 is that any reflected laser radiationis also deflected away from the emitting areas 36. Further, the layersof the bar stack between the bars 34 typically contain water coolingchannels (not shown) so the thermal energy of the reflected radiation isimmediately absorbed. The housing 79 and sealed window 83 shouldconstitute a sufficiently impermeable structure, so that any evaporatedresidue resulting from failure is confined within the structure for atime sufficient to detect the failure and remove power to the laserstack 32.

An alternative structure illustrated in the cross sectional view of FIG.8 places the pyrometer filter 84 on the bottom of the tilted window 83within the sealed housing so its failure is also confined to theinterior of the housing 79. A further alternative structure illustratedin the cross sectional view of FIG. 9 has housing sidewalls 86, 88 ofequal height. A wedge-shaped window 92 sits flat on the sidewalls 86, 88but the pyrometer filter 84 is tilted with respected to the emittedbeams 81 on the top face of the wedged window 92.

The anamorphic optics between the optical source and the wafer typicallyinclude anti-reflection coatings. With the invention, theanti-reflection and other coatings need to be designed not only for thelaser wavelength but also for the pyrometer wavelength.

Although the invention has been described for scanned line beams, it mayalso be applied to pulsed laser systems sequentially irradiatingadjacent portions of the substrate. However, the CW scanned operation isadvantageous in that the pyrometer is imaged with a scanned region that,from the view point of the pyrometer, is of temporally unvaryingtemperature.

With some redesign in the overall system, the wavelength filteringperformed by the PMUX interfaces can be replaced by selectivetransmission at the laser wavelength and selective reflection at thepyrometer wavelength. It may be possible to carry out the inventionwithout either the interleaver 42 or the polarization multiplexer 52 orwithout both of them. As one possible example, an optical elementsimilar to the reflective surfaces 54, 56 (or similar to one of them)may be employed to carry out the invention, that element not being partof a polarization multiplexer.

While the invention has been described in detail by specific referenceto preferred embodiments, it is understood that variations andmodifications thereof may be made without departing from the true spiritand scope of the invention.

1. A method of heating a wafer of a semiconductor material, comprising:generating laser radiation of a laser wavelength that is absorbed asheat by the semiconductor material; filtering from said laser radiationa pyrometer wavelength band at which said semiconductor material emitsblackbody radiation when heated by said laser radiation; passing saidlaser radiation through a beam splitter reflective at said laserwavelength and transmissive at said pyrometer wavelength band; focusingthe laser radiation reflected by the beam splitter to a line beam havinga narrow width that is a fraction of a diameter of the wafer; heating aconstantly moving narrow surface zone of said wafer corresponding tosaid narrow line beam width to a selected temperature on the order of1000 degrees C. for a time duration by scanning said line beam across asurface of the wafer at a selected scan rate that is sufficient to limitsaid time duration and limit a depth of said surface zone; imaging themoving zone of the wafer surface illuminated by the line beam onto thebeam splitter to provide a beam of wafer radiation emanating from themoving surface zone of the wafer; and passing the portion of said waferradiation lying within said pyrometer wavelength band through said beamsplitter to an optical detector responsive within said pyrometerwavelength band.
 2. The method of claim 1 wherein said laser radiationis produced from an array of discrete laser emitters, and said filteringfrom said laser radiation a pyrometer wavelength band comprisesreflecting the portion of said laser radiation lying within saidpyrometer wavelength band to zones between adjacent ones of theemitters.
 3. The method of claim 1 further comprising filtering outradiation of said laser wavelength from radiation traveling from saidbeam splitter to said optical detector.
 4. The method of claim 1 furthercomprising performing said focusing step and said imaging step in acommon optics.
 5. The method of claim 4 wherein said focusing step andsaid imaging step comprise passing the laser radiation through anoptical path of the common optics in a first forward direction andpassing the wafer radiation through said optical path in the oppositedirection.
 6. The method of claim 1 wherein said narrow width of saidline beam is on the order of 66 microns, and said laser radiationcomprises an optical beam having a power density on the order of 220kW/cm².
 7. The method of claim 1 where said laser wavelength is on theorder of 810 nm.
 8. The method of claim 7 wherein said pyrometerwavelength band is centered at about 1550 nm.
 9. The method of claim 7wherein said pyrometer wavelength band is centered at about 950 nm.